Quick Shooter

Injection molding just got a whole lot faster. No, not the part of the process that takes place on the molding machine. Those aspects of injection molding that really take time—tooling design, mold making, and startup—just got quicker. The ProtoMold Company in Maple Plain, MN (www.protomold.com) has given these upfront aspects of molding process a real jolt with its proprietary manufacturing process. "We can go from art to real molded parts in as little as a day," though a five-day lead time is routine, says company president and CEO Brad Cleveland. He calls the process "rapid injection molding."

In some ways, rapid injection molding has grown out of the rapid prototyping industry. Engineers have become accustomed to the idea that their CAD models can be turned into a physical prototype in a matter of days. But most rapid prototyping produces only an approximation of the end-use part, usually one not suited to functional testing. A limited portfolio of prototyping materials also means that the parts often can't be made in the same plastic as the finished product. And even when a close material match is available, plastic parts produced on prototyping machines lack the molded-in stresses and thermal history that influence the mechanical performance of a molded part.

Rapid tooling, or the use of rapid prototyping machinery to make mold inserts, can largely eliminate these difficulties. The inserts made on various rapid prototyping machines do differ from traditional production tools in terms of surface finish and cooling characteristics, but they can often turn out acceptable production or test parts in the end-use material. And they can reduce tooling lead times from a matter of weeks to days.

A different route

ProtoMold takes a different route to delivering molded parts quickly—one based on proprietary design software, high-speed CNC milling machines, and aluminum tools. What's the main difference between ProtoMold and those technologies that use rapid prototyping machines to build tooling inserts layer-by-layer? "We're faster," says Cleveland. He attributes some of the speed advantage to the one-step nature of CNC machining versus the multi-step processes needed to transform rapid prototyped parts into tooling inserts. More credit, however, goes to ProtoMold's software. The company has developed proprietary software to automate tasks that normally require hands-on work by tooling engineers. "Our core concept is eliminating the upfront engineering steps," Cleveland says.

To work with ProtoMold, users simply go to the web site and submit a CAD file of their part. Software generates a mold design—including parting line selection, gating, and ejector-pin layout. This initial design then goes back to the customer, via email in a matter of hours, for approval and any minor tweaking if necessary. Once the design is approved, ProtoMold software generates tool paths. The company then cuts the tools in aluminum using its three-axis, high-speed milling machine. It next molds the parts in-house and overnight mails them out. All of this takes place in as little as a day, though Cleveland says parts made to a more leisurely timetable would cost less.

Speed limits

For all its speed, ProtoMold's process does have a few limitations. Size-wise, the company's CNC and machinery can currently handle parts with depths less than 1.5 inches from the parting line—for a 3-inch maximum depth for parts centered in the cavity. And the company's process doesn't currently support parts that need side actions. "We are strictly a straight pull house," Cleveland says. Fine features, particularly those with high-aspect ratios, can cause problems too because of ProtoMold's reliance on milling cutters rather than EDM machines. Cleveland cites a limit, for example, of about 0.016-inch diameter on round features and a 0.015-inch limit on radii. More complete design guidelines are available on the company's website, and the company's software will flag any unsuitable designs. Finally, ProtoMold's process doesn't currently support insert molding or overmolding.

It's worth noting, however, that these process limitations may ease over time. Cleveland says the ability to support side-actions, for example, will be ready this month. "That limitation is strictly software related," he says. And he adds that insert molding and overmolding will follow suit.

Over the past five years, ProtoMold has supplied rapid injection molded parts to OEMs from a variety of industries—including medical, telecommunications, and consumer electronics. One power user has been John Deere & Co.'s agriculture equipment group in Moline, IL. Mike Friestad, an engineer there who designs seeding systems, reports that the company has used the service for dozens of parts and that the system is "as fast as promised."

Deere has used ProtoMold to produce a handful of final production parts, but the company mostly uses rapid injection molding as a way to get truly functional prototypes for mechanical testing. "We still end up going to hard tooling in most cases," he says. The reason why partly comes down to the production volumes and durability. Deere sometimes makes enough parts to justify multi-cavity tools, Friestad says. And even with single cavity tools, hard tooling lasts longer, particularly in the face of harsh, glass-filled engineering resins.

Hard tooling usually wins out for a couple of other reasons, too. One is a matter of detail. ProtoMold's limitation regarding detailed features and radii sometimes takes it out of the running for the finished part. For example, some of Deere's parts require numbering that ProtoMold's process can't add, Friestad says. The other reason has to do with subtle differences between parts molded in aluminum and those molded in steel. "Even if they come from the same lot of material, parts molded in different materials aren't necessarily the same," Friestad notes. These differences are often small and may not matter.

But they can. On a recent glass-filled PBT seeder gearbox housing, for example, Friestad says Deere found that the parts made on the aluminum tooling exhibited more warpage than those out of the final steel production tools. "We're not really sure why," he says. He speculates, however, that the different tooling materials created different cooling conditions—and ultimately affected molded-in stresses.

So why not just use a rapid prototyping machine to get the functional prototypes in a hurry? Freistad notes that rapid prototyping machines don't always offer a material equivalent to the end-use plastic. And when they do, the differences between aluminum- and steel-molded parts pale in comparison to the differences between injection-molded and rapid-prototyped parts.

One limitation that hasn't bothered Friestad much has been the reliance on straight pull molds only. Designing for ProtoMold's system, while it may rule out some part geometries, does impose some discipline than can help contain costs. Friestad says he intentionally sets out to design parts that can run in a straight pull mold because these molds cost less to build and to run. "It's just good design practice to design without core pulls whenever you can," he says. "And that fits nicely with ProtoMold's process."

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